The Mechanical Properties of Wood by Samuel J. Record

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The term _hardness_ is used in two senses, namely: (1)resistance to indentation, and (2) resistance to abrasion orscratching. In the latter sense hardness combined with toughnessis a measure of the wearing ability of wood and is an importantconsideration in the use of wood for floors, paving blocks,bearings, and rollers. While resistance to indentation isdependent mostly upon the density of the wood, the wearingqualities may be governed by other factors such as toughness,and the size, cohesion, and arrangement of the fibres. In usefor floors, some woods tend to compact and wear smooth, whileothers become splintery and rough. This feature is affected tosome extent by the manner in which the wood is sawed; thusedge-grain pine flooring is much better than flat-sawn foruniformity of wear.

Tests for either form of hardness are of comparative value only.Tests for indentation are commonly made by penetrations of thematerial with a steel punch or ball.[16] Tests for abrasion aremade by wearing down wood with sandpaper or by means of a sandblast.

_Cleavability_ is the term used to denote the facility withwhich wood is split. A splitting stress is one in which theforces act normally like a wedge. (See Fig. 21.) The plane ofcleavage is parallel to the grain, either radially ortangentially.

This property of wood is very important in certain uses such asfirewood, fence rails, billets, and squares. Resistance tosplitting or low cleavability is desirable where wood must holdnails or screws, as in box-making. Wood usually splits morereadily along the radius than parallel to the growth ringsthough exceptions occur, as in the case of cross grain.

Splitting involves transverse tension, but only a portion of thefibres are under stress at a time. A wood of little stiffnessand strong cohesion across the grain is difficult to split,while one with great stiffness, such as longleaf pine, is easilysplit. The form of the grain and the presence of knots greatlyaffect this quality.

Wood is an organic product--a structure of infinite variation ofdetail and design.[17] It is on this account that no two woodsare alike--in reality no two specimens from the same log areidentical. There are certain properties that characterize eachspecies, but they are subject to considerable variation. Oak,for example, is considered hard, heavy, and strong, but somepieces, even of the same species of oak, are much harder,heavier, and stronger than others. With hickory are associatedthe properties of great strength, toughness, and resilience, butsome pieces are comparatively weak and brash and ill-suited forthe exacting demands for which good hickory is peculiarlyadapted.

It follows that no definite value can be assigned to theproperties of any wood and that tables giving average results oftests may not be directly applicable to any individual stick.With sufficient knowledge of the intrinsic factors affecting theresults it becomes possible to infer from the appearance ofmaterial its probable variation from the average. As yet toolittle is known of the relation of structure and chemicalcomposition to the mechanical and physical properties to permitmore than general conclusions.

RATE OF GROWTH

To understand the effect of variations in the rate of growth itis first necessary to know how wood is formed. A tree increasesin diameter by the formation, between the old wood and the innerbark, of new woody layers which envelop the entire stem, livingbranches, and roots. Under ordinary conditions one layer isformed each year and in cross section as on the end of a logthey appear as rings--often spoken of as _annual rings_. Thesegrowth layers are made up of wood cells of various kinds, butfor the most part fibrous. In timbers like pine, spruce,hemlock, and other coniferous or softwood species the wood cellsare mostly of one kind, and as a result the material is muchmore uniform in structure than that of most hardwoods. (SeeFrontispiece.) There are no vessels or pores in coniferous woodsuch as one sees so prominently in oak and ash, for example.(See Fig. 22.)

The structure of the hardwoods is more complex. They are more orless filled with vessels, in some cases (oak, chestnut, ash)quite large and distinct, in others (buckeye, poplar, gum) toosmall to be seen plainly without a small hand lens. Indiscussing such woods it is customary to divide them into twolarge classes--_ring-porous_ and _diffuse-porous_. (See Fig.22.) In ring-porous species, such as oak, chestnut, ash, blacklocust, catalpa, mulberry, hickory, and elm, the larger vesselsor pores (as cross sections of vessels are called) becomelocalized in one part of the growth ring, thus forming a regionof more or less open and porous tissue. The rest of the ring ismade up of smaller vessels and a much greater proportion of woodfibres. These fibres are the elements which give strength andtoughness to wood, while the vessels are a source of weakness.

In diffuse-porous woods the pores are scattered throughout thegrowth ring instead of being collected in a band or row.Examples of this kind of wood are gum, yellow poplar, birch,maple, cottonwood, basswood, buckeye, and willow. Some species,such as walnut and cherry, are on the border between the twoclasses, forming a sort of intermediate group.

If one examines the smoothly cut end of a stick of almost anykind of wood, he will note that each growth ring is made up oftwo more or less well-defined parts. That originally nearest thecentre of the tree is more open textured and almost invariablylighter in color than that near the outer portion of the ring.The inner portion was formed early in the season, when growthwas comparatively rapid and is known as _early wood_ (alsospring wood); the outer portion is the _late wood_, beingproduced in the summer or early fall. In soft pines there is notmuch contrast in the different parts of the ring, and as aresult the wood is very uniform in texture and is easy to work.In hard pine, on the other hand, the late wood is very dense andis deep-colored, presenting a very decided contrast to the soft,straw-colored early wood. (See Fig. 23.) In ring-porous woodseach season's growth is always well defined, because the largepores of the spring abut on the denser tissue of the fallbefore. In the diffuse-porous, the demarcation between rings isnot always so clear and in not a few cases is almost, if notentirely, invisible to the unaided eye. (See Fig. 22.)

[Illustration: FIG. 23.--Cross section of longleaf pine showingseveral growth rings with variations in the width of thedark-colored late wood. Seven resin ducts are visible. X 33._Photomicrograph by U.S. Forest Service_]

If one compares a heavy piece of pine with a light specimen itwill be seen at once that the heavier one contains a largerproportion of late wood than the other, and is thereforeconsiderably darker. The late wood of all species is denser thanthat formed early in the season, hence the greater theproportion of late wood the greater the density and strength.When examined under a microscope the cells of the late wood areseen to be very thick-walled and with very small cavities, whilethose formed first in the season have thin walls and largecavities. The strength is in the walls, not the cavities. Inchoosing a piece of pine where strength or stiffness is theimportant consideration, the principal thing to observe is thecomparative amounts of early and late wood. The width of ring,that is, the number per inch, is not nearly so important as theproportion of the late wood in the ring.

It is not only the proportion of late wood, but also itsquality, that counts. In specimens that show a very largeproportion of late wood it may be noticeably more porous andweigh considerably less than the late wood in pieces thatcontain but little. One can judge comparative density, andtherefore to some extent weight and strength, by visualinspection.

The conclusions of the U.S. Forest Service regarding the effectof rate of growth on the properties of Douglas fir aresummarized as follows:

"1. In general, rapidly grown wood (less than eight rings perinch) is relatively weak. A study of the individual tests uponwhich the average points are based shows, however, that when itis not associated with light weight and a small proportion ofsummer wood, rapid growth is not indicative of weak wood.

"2. An average rate of growth, indicated by from 12 to 16 ringsper inch, seems to produce the best material.

"3. In rates of growths lower than 16 rings per inch, theaverage strength of the material decreases, apparentlyapproaching a uniform condition above 24 rings per inch. In suchslow rates of growth the texture of the wood is very uniform,and naturally there is little variation in weight or strength.

"An analysis of tests on large beams was made to ascertain ifaverage rate of growth has any relation to the mechanicalproperties of the beams. The analysis indicated conclusivelythat there was no such relation. Average rate of growth [withoutconsideration also of density], therefore, has littlesignificance in grading structural timber."[18] This is becauseof the wide variation in the percentage of late wood indifferent parts of the cross section.

[Footnote 18: Bul. 88: Properties and uses of Douglas fir, p.29.]

Experiments seem to indicate that for most species there is arate of growth which, in general, is associated with thegreatest strength, especially in small specimens. For eightconifers it is as follows:[19]

No satisfactory explanation can as yet be given for the realcauses underlying the formation of early and late wood. Severalfactors may be involved. In conifers, at least, rate of growthalone does not determine the proportion of the two portions ofthe ring, for in some cases the wood of slow growth is very hardand heavy, while in others the opposite is true. The quality ofthe site where the tree grows undoubtedly affects the characterof the wood formed, though it is not possible to formulate arule governing it. In general, however, it may be said thatwhere strength or ease of working is essential, woods ofmoderate to slow growth should be chosen. But in choosing aparticular specimen it is not the width of ring, but theproportion and character of the late wood which should govern.

In the case of the ring-porous hardwoods there seems to exist apretty definite relation between the rate of growth of timberand its properties. This may be briefly summed up in the generalstatement that the more rapid the growth or the wider the ringsof growth, the heavier, harder, stronger, and stiffer the wood.This, it must be remembered, applies only to ring-porous woodssuch as oak, ash, hickory, and others of the same group, and is,of course, subject to some exceptions and limitations.

In ring-porous woods of good growth it is usually the middleportion of the ring in which the thick-walled, strength-givingfibres are most abundant. As the breadth of ring diminishes,this middle portion is reduced so that very slow growth producescomparatively light, porous wood composed of thin-walled vesselsand wood parenchyma. In good oak these large vessels of theearly wood occupy from 6 to 10 per cent of the volume of thelog, while in inferior material they may make up 25 per cent ormore. The late wood of good oak, except for radial grayishpatches of small pores, is dark colored and firm, and consistsof thick-walled fibres which form one-half or more of the wood.In inferior oak, such fibre areas are much reduced both inquantity and quality. Such variation is very largely the resultof rate of growth.

Wide-ringed wood is often called "second-growth," because thegrowth of the young timber in open stands after the old treeshave been removed is more rapid than in trees in the forest, andin the manufacture of articles where strength is an importantconsideration such "second-growth" hardwood material ispreferred. This is particularly the case in the choice ofhickory for handles and spokes. Here not only strength, buttoughness and resilience are important. The results of a seriesof tests on hickory by the U.S. Forest Service show that "thework or shock-resisting ability is greatest in wide-ringed woodthat has from 5 to 14 rings per inch, is fairly constant from 14to 38 rings, and decreases rapidly from 38 to 47 rings. Thestrength at maximum load is not so great with the mostrapid-growing wood; it is maximum with from 14 to 20 rings perinch, and again becomes less as the wood becomes more closelyringed. The natural deduction is that wood of first-classmechanical value shows from 5 to 20 rings per inch and thatslower growth yields poorer stock. Thus the inspector or buyerof hickory should discriminate against timber that has more than20 rings per inch. Exceptions exist, however, in the case ofnormal growth upon dry situations, in which the slow-growingmaterial may be strong and tough."[20]

[Footnote 20: Bul. 80: The commercial hickories, pp. 48-50.]

The effect of rate of growth on the qualities of chestnut woodis summarized by the same authority as follows: "When the ringsare wide, the transition from spring wood to summer wood isgradual, while in the narrow rings the spring wood passes intosummer wood abruptly. The width of the spring wood changes butlittle with the width of the annual ring, so that the narrowingor broadening of the annual ring is always at the expense of thesummer wood. The narrow vessels of the summer wood make itricher in wood substance than the spring wood composed of widevessels. Therefore, rapid-growing specimens with wide rings havemore wood substance than slow-growing trees with narrow rings.Since the more the wood substance the greater the weight, andthe greater the weight the stronger the wood, chestnuts withwide rings must have stronger wood than chestnuts with narrowrings. This agrees with the accepted view that sprouts (whichalways have wide rings) yield better and stronger wood thanseedling chestnuts, which grow more slowly in diameter."[21]

[Footnote 21: Bul. 53: Chestnut in southern Maryland, pp.20-21.]

In diffuse-porous woods, as has been stated, the vessels orpores are scattered throughout the ring instead of collected inthe early wood. The effect of rate of growth is, therefore, notthe same as in the ring-porous woods, approaching more nearlythe conditions in the conifers. In general it may be stated thatsuch woods of medium growth afford stronger material than whenvery rapidly or very slowly grown. In many uses of wood,strength is not the main consideration. If ease of working isprized, wood should be chosen with regard to its uniformity oftexture and straightness of grain, which will in most casesoccur when there is little contrast between the late wood of oneseason's growth and the early wood of the next.

HEARTWOOD AND SAPWOOD

Examination of the end of a log of many species reveals adarker-colored inner portion--the _heartwood_, surrounded by alighter-colored zone--the _sapwood_. In some instances thisdistinction in color is very marked; in others, the contrast isslight, so that it is not always easy to tell where one leavesoff and the other begins. The color of fresh sapwood is alwayslight, sometimes pure white, but more often with a decided tingeof green or brown.

Sapwood is comparatively new wood. There is a time in the earlyhistory of every tree when its wood is all sapwood. Itsprincipal functions are to conduct water from the roots to theleaves and to store up and give back according to the season thefood prepared in the leaves. The more leaves a tree bears andthe more thrifty its growth, the larger the volume of sapwoodrequired, hence trees making rapid growth in the open havethicker sapwood for their size than trees of the same speciesgrowing in dense forests. Sometimes trees grown in the open maybecome of considerable size, a foot or more in diameter, beforeany heartwood begins to form, for example, in second-growthhickory, or field-grown white and loblolly pines.

As a tree increases in age and diameter an inner portion of thesapwood becomes inactive and finally ceases to function. Thisinert or dead portion is called heartwood, deriving its namesolely from its position and not from any vital importance tothe tree, as is shown by the fact that a tree can thrive withits heart completely decayed. Some, species begin to formheartwood very early in life, while in others the change comesslowly. Thin sapwood is characteristic of such trees aschestnut, black locust, mulberry, Osage orange, and sassafras,while in maple, ash, gum, hickory, hackberry, beech, andloblolly pine, thick sapwood is the rule.

There is no definite relation between the annual rings of growthand the amount of sapwood. Within the same species thecross-sectional area of the sapwood is roughly proportional tothe size of the crown of the tree. If the rings are narrow, moreof them are required than where they are wide. As the tree getslarger, the sapwood must necessarily become thinner or increasematerially in volume. Sapwood is thicker in the upper portion ofthe trunk of a tree than near the base, because the age and thediameter of the upper sections are less.

When a tree is very young it is covered with limbs almost, ifnot entirely, to the ground, but as it grows older some or allof them will eventually die and be broken off. Subsequent growthof wood may completely conceal the stubs which, however, willremain as knots. No matter how smooth and clear a log is on theoutside, it is more or less knotty near the middle. Consequentlythe sapwood of an old tree, and particularly of a forest-growntree, will be freer from knots than the heartwood. Since in mostuses of wood, knots are defects that weaken the timber andinterfere with its ease of working and other properties, itfollows that sapwood, because of its position in the tree, mayhave certain advantages over heartwood.

It is really remarkable that the inner heartwood of old treesremains as sound as it usually does, since in many cases it ishundreds of years, and in a few instances thousands of years,old. Every broken limb or root, or deep wound from fire,insects, or falling timber, may afford an entrance for decay,which, once started, may penetrate to all parts of the trunk.The larvae of many insects bore into the trees and their tunnelsremain indefinitely as sources of weakness. Whatever advantages,however, that sapwood may have in this connection are due solelyto its relative age and position.

If a tree grows all its life in the open and the conditions ofsoil and site remain unchanged, it will make its most rapidgrowth in youth, and gradually decline. The annual rings ofgrowth are for many years quite wide, but later they becomenarrower and narrower. Since each succeeding ring is laid downon the outside of the wood previously formed, it follows thatunless a tree materially increases its production of wood fromyear to year, the rings must necessarily become thinner. As atree reaches maturity its crown becomes more open and the annualwood production is lessened, thereby reducing still more thewidth of the growth rings. In the case of forest-grown trees somuch depends upon the competition of the trees in their strugglefor light and nourishment that periods of rapid and slow growthmay alternate. Some trees, such as southern oaks, maintain thesame width of ring for hundreds of years. Upon the whole,however, as a tree gets larger in diameter the width of thegrowth rings decreases.

It is evident that there may be decided differences in the grainof heartwood and sapwood cut from a large tree, particularly onethat is overmature. The relationship between width of growthrings and the mechanical properties of wood is discussed underRate of Growth. In this connection, however, it may be statedthat as a general rule the wood laid on late in the life of atree is softer, lighter, weaker, and more even-textured thanthat produced earlier. It follows that in a large log thesapwood, because of the time in the life of the tree when it wasgrown, may be inferior in hardness, strength, and toughness toequally sound heartwood from the same log.

After exhaustive tests on a number of different woods the U.S.Forest Service concludes as follows: "Sapwood, except that fromold, overmature trees, is as strong as heartwood, other thingsbeing equal, and so far as the mechanical properties go shouldnot be regarded as a defect."[22] Careful inspection of theindividual tests made in the investigation fails to reveal anyrelation between the proportion of sapwood and the breakingstrength of timber.

[Footnote 22: Bul. 108: Tests of structural timber, p. 35.]

In the study of the hickories the conclusion was: "There is anunfounded prejudice against the heartwood. Specifications placewhite hickory, or sapwood, in a higher grade than red hickory,or heartwood, though there is no inherent difference instrength. In fact, in the case of large and old hickory trees,the sapwood nearest the bark is comparatively weak, and the bestwood is in the heart, though in young trees of thrifty growththe best wood is in the sap."[23] The results of tests fromselected pieces lying side by side in the same tree, and alsothe average values for heartwood and sapwood in shipments of thecommercial hickories without selection, show conclusively that"the transformation of sapwood into heartwood does not affecteither the strength or toughness of the wood.... It is true,however, that sapwood is usually more free from latent defectsthan heartwood."[24]

[Footnote 23: Bul. 80: The commercial hickories, p. 50.]

[Footnote 24: _Loc. cit._]

Specifications for paving blocks often require that longleafpine be 90 per cent heart. This is on the belief that sapwood isnot only more subject to decay, but is also weaker thanheartwood. In reality there is no sound basis for discriminationagainst sapwood on account of strength, provided otherconditions are equal. It is true that sapwood will not resistdecay as long as heartwood, if both are untreated withpreservatives. It is especially so of woods with deep-coloredheartwood, and is due to infiltrations of tannins, oils, andresins, which make the wood more or less obnoxious todecay-producing fungi. If, however, the timbers are to betreated, sapwood is not a defect; in fact, because of therelative ease with which it can be impregnated withpreservatives it may be made more desirable than heartwood.[25]

[Footnote 25: Although the factor of heart or sapwood does notinfluence the mechanical properties of the wood and there isusually no difference in structure observable under themicroscope, nevertheless sapwood is generally decidedlydifferent from heartwood in its physical properties. It driesbetter and more easily than heartwood, usually with lessshrinkage and little checking or honeycombing. This isespecially the case with the more refractory woods, such aswhite oaks and _Eucalyptus globulus_ and _viminalis_. It isusually much more permeable to air, even in green wood, notablyso in loblolly pine and even in white oak. As already stated, itis much more subject to decay. The sapwood of white oak may beimpregnated with creosote with comparative ease, while theheartwood is practically impenetrable. These facts indicate adifference in its chemical nature.--H.D. Tiemann.]

In specifications for structural timbers reference is sometimesmade to "boxheart," meaning the inclusion of the pith or centreof the tree within a cross section of the timber. From numerousexperiments it appears that the position of the pith does notbear any relation to the strength of the material. Since mostseason checks, however, are radial, the position of the pith mayinfluence the resistance of a seasoned beam to horizontal shear,being greatest when the pith is located in the middle half ofthe section.[26]

[Footnote 26: Bul. 108, U.S. Forest Service, p. 36.]

WEIGHT, DENSITY, AND SPECIFIC GRAVITY

From data obtained from a large number of tests on the strengthof different woods it appears that, other things being equal,the crushing strength parallel to the grain, fibre stress atelastic limit in bending, and shearing strength along the grainof wood vary in direct proportion to the weight of dry wood perunit of volume when green. Other strength values followdifferent laws. The hardness varies in a slightly greater ratiothan the square of the density. The work to the breaking pointincreases even more rapidly than the cube of density. Themodulus of rupture in bending lies between the first power andthe square of the density. This, of course, is true only in casethe greater weight is due to increase in the amount of woodsubstance. A wood heavy with resin or other infiltratedsubstance is not necessarily stronger than a similar specimenfree from such materials. If differences in weight are due todegree of seasoning, in other words, to the relative amounts ofwater contained, the rules given above will of course not hold,since strength increases with dryness. But of given specimens ofpine or of oak, for example, in the green condition, thecomparative strength may be inferred from the weight. It is notpermissible, however, to compare such widely different woods asoak and pine on a basis of their weights.[27]

[Footnote 27: The oaks for some unknown reason fall below thenormal strength for weight, whereas the hickories rise above.Certain other woods also are somewhat exceptional to the normalrelation of strength and density.]

The weight of wood substance, that is, the material whichcomposes the walls of the fibres and other cells, is practicallythe same in all species, whether pine, hickory, or cottonwood,being a little greater than half again as heavy as water. Itvaries slightly from beech sapwood, 1.50, to Douglas firheartwood, 1.57, averaging about 1.55 at 30 deg. to 35 deg. C., in termsof water at its greatest density 4 deg. C. The reason any woodfloats is that the air imprisoned in its cavities buoys it up.When this is displaced by water the wood becomes water-loggedand sinks. Leaving out of consideration infiltrated substances,the reason a cubic foot of one kind of dry wood is heavier thanthat of another is because it contains a greater amount of woodsubstance. ~Density~ is merely the weight of a unit of volume,as 35 pounds per cubic foot, or 0.56 grams per cubic centimetre.~Specific gravity~ or relative density is the ratio of thedensity of any material to the density of distilled water at 4 deg.C. (39.2 deg. F.). A cubic foot of distilled water at 4 deg. C. weighs62.43 pounds. Hence the specific gravity of a piece of wood witha density of 35 pounds is 35 / 62.43 = 0.561. To find the weightper cubic foot when the specific gravity is given, simplymultiply by 62.43. Thus, 0.561 X 62.43 = 35. In the metricsystem, since the weight of a cubic centimetre of pure water isone gram, the density in grams per cubic centimetre has the samenumerical value as the specific gravity.

Since the amount of water in wood is extremely variable itusually is not satisfactory to refer to the density of greenwood. For scientific purposes the density of "oven-dry" wood isused; that is, the wood is dried in an oven at a temperature of100 deg.C. (212 deg.F.) until a constant weight is attained. Forcommercial purposes the weight or density of air-dry or"shipping-dry" wood is used. This is usually expressed in poundsper thousand board feet, a board foot being considered asone-twelfth of a cubic foot.

Wood shrinks greatly in drying from the green to the oven-drycondition. (See Table XIV.) Consequently a block of woodmeasuring a cubic foot when green will measure considerably lesswhen oven-dry. It follows that the density of oven-dry wood doesnot represent the weight of the dry wood substance in a cubicfoot of green wood. In other words, it is not the weight of acubic foot of green wood minus the weight of the water which itcontains. Since the latter is often a more convenient figure touse and much easier to obtain than the weight of oven-dry wood,it is commonly expressed in tables of "specific gravity ordensity of dry wood."

This weight divided by 62.43 gives the specific gravity pergreen volume. It is purely a fictitious quantity. To convertthis figure into actual density or specific gravity of the drywood, it is necessary to know the amount of shrinkage in volume.If S is the percentage of shrinkage from the green to theoven-dry condition, based on the green volume; D, the density ofthe dry wood per cubic foot while green; and d the actual Ddensity of oven-dry wood, then ---------- = d. 1 - .0 S

This relation becomes clearer from the following analysis:Taking V and W as the volume and weight, respectively, whengreen, and v and w as the corresponding volume and weight when w W V - voven-dry, then, d = --- ; D = --- ; S = ------- X 100, and v V V V - vs = ------- X 100, in which S is the percentage of shrinkage vfrom the green to the oven-dry condition, based on the greenvolume, and s the same based on the oven-dry volume.

In tables of specific gravity or density of wood it shouldalways be stated whether the dry weight per unit of volume whengreen or the dry weight per unit of volume when dry is intended,since the shrinkage in volume may vary from 6 to 50 per cent,though in conifers it is usually about 10 per cent, and inhardwoods nearer 15 per cent. (See Table XIV.)

COLOR

In species which show a distinct difference between heartwoodand sapwood the natural color of heartwood is invariably darkerthan that of the sapwood, and very frequently the contrast isconspicuous. This is produced by deposits in the heartwood ofvarious materials resulting from the process of growth,increased possibly by oxidation and other chemical changes,which usually have little or no appreciable effect on themechanical properties of the wood. (See HEARTWOOD AND SAPWOOD,above.) Some experiments[28] on very resinous longleaf pinespecimens, however, indicate an increase in strength. This isdue to the resin which increases the strength when dry. Spruceimpregnated with crude resin and dried is greatly increased instrength thereby.

Since the late wood of a growth ring is usually darker in colorthan the early wood, this fact may be used in judging thedensity, and therefore the hardness and strength of thematerial. This is particularly the case with coniferous woods.In ring-porous woods the vessels of the early wood notinfrequently appear on a finished surface as darker than thedenser late wood, though on cross sections of heartwood thereverse is commonly true. Except in the manner just stated thecolor of wood is no indication of strength.

Abnormal discoloration of wood often denotes a diseasedcondition, indicating unsoundness. The black check in westernhemlock is the result of insect attacks.[29] The reddish-brownstreaks so common in hickory and certain other woods are mostlythe result of injury by birds.[30] The discoloration is merelyan indication of an injury, and in all probability does not ofitself affect the properties of the wood. Certain rot-producingfungi impart to wood characteristic colors which thus becomecriterions of weakness. Ordinary sap-staining is due to fungousgrowth, but does not necessarily produce a weakening effect.[31]

_Cross grain_ is a very common defect in timber. One form of itis produced in lumber by the method of sawing and has noreference to the natural arrangement of the wood elements. Thusif the plane of the saw is not approximately parallel to theaxis of the log the grain of the lumber cut is not parallel tothe edges and is termed diagonal. This is likely to occur wherethe logs have considerable taper, and in this case may beproduced if sawed parallel to the axis of growth instead ofparallel to the growth rings.

Lumber and timber with diagonal grain is always weaker thanstraight-grained material, the extent of the defect varying withthe degree of the angle the fibres make with the axis of thestick. In the vicinity of large knots the grain is likely to becross. The defect is most serious where wood is subjected toflexure, as in beams.

_Spiral grain_ is a very common defect in a tree, and whenexcessive renders the timber valueless for use except in theround. It is produced by the arrangement of the wood fibres in aspiral direction about the axis instead of exactly vertical.Timber with spiral grain is also known as "torse wood." Spiralgrain usually cannot be detected by casual inspection of astick, since it does not show in the so-called visible grain ofthe wood, by which is commonly meant a sectional view of theannual rings of growth cut longitudinally. It is accordinglyvery easy to allow spiral-grained material to pass inspection,thereby introducing an element of weakness in a structure.

There are methods for readily detecting spiral grain. Thesimplest is that of splitting a small piece radially. It isnecessary, of course, that the split be radial, that is, in aplane passing through the axis of the log, and not tangentially.In the latter case it is quite probable that the wood wouldsplit straight, the line of cleavage being between the growthrings.

In inspection, the elements to examine are the rays. In the caseof oak and certain other hardwoods these rays are so large thatthey are readily seen not only on a radial surface, but on thetangential as well. On the former they appear as flakes, on thelatter as short lines. Since these rays are between the fibresit naturally follows that they will be vertical or inclinedaccording as the tree is straight-grained or spiral-grained.While they are not conspicuous in the softwoods, they can beseen upon close scrutiny, and particularly so if a small handmagnifier is used.

When wood has begun to dry and check it is very easy to seewhether or not it is straight- or spiral-grained, since thechecks will for the most part follow along the rays. If oneexamines a row of telephone poles, for example, he will probablyfind that most of them have checks running spirally around them.If boards were sawed from such a pole after it was badly checkedthey would fall to pieces of their own weight. The only way toget straight material would be to split it out.

It is for this reason that split billets and squares arestronger than most sawed material. The presence of the spiralgrain has little, if any, effect on the timber when it is usedin the round, but in sawed material the greater the pitch of thespiral the greater is the defect.

KNOTS

_Knots_ are portions of branches included in the wood of thestem or larger branch. Branches originate as a rule from thecentral axis of a stem, and while living increase in size by theaddition of annual woody layers which are a continuation ofthose of the stem. The included portion is irregularly conicalin shape with the tip at the pith. The direction of the fibre isat right angles or oblique to the grain of the stem, thusproducing local cross grain.

During the development of a tree most of the limbs, especiallythe lower ones, die, but persist for a time--often for years.Subsequent layers of growth of the stem are no longer intimatelyjoined with the dead limb, but are laid around it. Hence deadbranches produce knots which are nothing more than pegs in ahole, and likely to drop out after the tree has been sawed intolumber. In grading lumber and structural timber, knots areclassified according to their form, size, soundness, and thefirmness with which they are held in place.[32]

Knots materially affect checking and warping, ease in working,and cleavability of timber. They are defects which weaken timberand depreciate its value for structural purposes where strengthis an important consideration. The weakening effect is much moreserious where timber is subjected to bending and tension thanwhere under compression. The extent to which knots affect thestrength of a beam depends upon their position, size, number,direction of fibre, and condition. A knot on the upper side iscompressed, while one on the lower side is subjected to tension.The knot, especially (as is often the case) if there is a seasoncheck in it, offers little resistance to this tensile stress.Small, knots, however, may be so located in a beam along theneutral plane as actually to increase the strength by tending toprevent longitudinal shearing. Knots in a board or plank areleast injurious when they extend through it at right angles toits broadest surface. Knots which occur near the ends of a beamdo not weaken it. Sound knots which occur in the central portionone-fourth the height of the beam from either edge are notserious defects.

Extensive experiments by the U.S. Forest Service[33] indicatethe following effects of knots on structural timbers:

[Footnote 33: Bul. 108, pp. 52 _et seq._]

(1) Knots do not materially influence the stiffness ofstructural timber.

(2) Only defects of the most serious character affect theelastic limit of beams. Stiffness and elastic strength are moredependent upon the quality of the wood fibre than upon defectsin the beam.

(3) The effect of knots is to reduce the difference between thefibre stress at elastic limit and the modulus of rupture ofbeams. The breaking strength is very susceptible to defects.

(4) Sound knots do not weaken wood when subject to compressionparallel to the grain.[34]

A common defect in standing timber results from radial splitswhich extend inward from the periphery of the tree, and almost,if not always, near the base. It is most common in trees whichsplit readily, and those with large rays and thin bark. Theprimary cause of the splitting is frost, and various theorieshave been advanced to explain the action.

R. Hartig[35] believes that freezing forces out a part of theimbibition water of the cell walls, thereby causing the wood toshrink, and if the interior layers have not yet been cooled,tangential strains arise which finally produce radial clefts.

[Footnote 35: Hartig, R.: The diseases of trees (trans. bySomerville and Ward), London and New York, 1894, pp. 282-294.]

Another theory holds that the water is not driven out of thecell walls, but that difference in temperature conditions ofinner and outer layers is itself sufficient to set up thestrains, resulting in splitting. An air temperature of 14 deg.F. orless is considered necessary to produce frost splits.

A still more recent theory is that of Busse[36] who considersthe mechanical action of the wind a very important factor. Heobserved: (_a_) Frost splits sometimes occur at highertemperatures than 14 deg.F. (_b_) Most splits take place shortlybefore sunrise, _i.e._, at the time of lowest air and soiltemperature; they are never heard to take place at noon,afternoon, or evening. (_c_) They always occur between two rootsor between the collars of two roots, (_d_) They are mostfrequent in old, stout-rooted, broad-crowned trees; in youngerstands it is always the stoutest members that are found withfrost splits, while in quite young stands they are altogetherabsent, (_e_) Trees on wet sites are most liable to splits, dueto difference in wood structure, just as difference in woodstructure makes different species vary in this regard. (_f_)Frost splits are most numerous less than three feet above theground.

When a tree is swayed by the wind the roots are counteractingforces, and the wood fibres are tested in tension andcompression by the opposing forces; where the roots exercisetension stresses most effectively the effect of compressionstresses is at a minimum; only where the pressure is in excessof the tension, _i.e._, between the roots, can a separation ofthe fibre result. Hence, when by frost a tension on the entireperiphery is established, and the wind localizes additionalstrains, failure occurs. The stronger the compression andtension, the severer the strains and the oftener failures occur.The occurrence of reports of frost splits on wind-still days isbelieved by Busse to be due to the opening of old frost splitswhere the tension produced by the frost alone is sufficient.

Frost splits may heal over temporarily, but usually open upagain during the following winter. The presence of old splits isoften indicated by a ridge of callous, the result of thecambium's effort to occlude the wound. Frost splits not onlyaffect the value of lumber, but also afford an entrance into theliving tree for disease and decay.

SHAKES, GALLS, PITCH POCKETS

_Heart shake_ occurs in nearly all overmature timber, being morefrequent in hardwoods (especially oak) than in conifers. Intypical heart shake the centre of the hole shows indications ofbecoming hollow and radial clefts of varying size extend outwardfrom the pith, being widest inward. It frequently affects onlythe butt log, but may extend to the entire hole and even thelarger branches. It usually results from a shrinkage of theheartwood due probably to chemical changes in the wood.

When it consists of a single cleft extending across the pith itis termed _simple heart shake_. Shake of this character instraight-grained trees affects only one or two central boardswhen cut into lumber, but in spiral-grained timber the damage ismuch greater. When shake consists of several radial clefts it istermed _star shake_. In some instances one or more of theseclefts may extend nearly to the bark. In felled or convertedtimber clefts due to heart shake may be distinguished fromseasoning cracks by the darker color of the exposed surfaces.Such clefts, however, tend to open up more and more as thetimber seasons.

_Cup_ or _ring shake_ results from the pulling apart of two ormore growth rings. It is one of the most serious defects towhich sound timber is subject, as it seriously reduces thetechnical properties of wood. It is very common in sycamore andin western larch, particularly in the butt portion. Itsoccurrence is most frequent at the junction of two growth layersof very unequal thickness. Consequently it is likely to occur intrees which have grown slowly for a time, then abruptlyincreased, due to improved conditions of light and food, as inthinning. Old timber is more subject to it than young trees. Thedamage is largely confined to the butt log. Cup shake is oftenassociated with other forms of shake, and not infrequently showstraces of decay.

The causes of cup shake are uncertain. The swaying action of thewind may result in shearing apart the growth layers, especiallyin trees growing in exposed places. Frost may in some instancesbe responsible for cup shake or at least a contributing factor,although trees growing in regions free from frost often havering shake. Shrinkage of the heartwood may be concentric as wellas radial in its action, thus producing cup shake instead of, orin connection with, heart shake.

A local defect somewhat similar in effect to cup shake is knownas _rind gall_. If the cambium layer is exposed by the removalof the entire bark or rind it will die. Subsequent growth overthe damaged portion does not cohere with the wood previouslyformed by the old cambium. The defect resulting is termed rindgall. The most common causes of it are fire, gnawing, blazing,chipping, sun scald, lightning, and abrasions.

_Heart break_ is a term applied to areas of compression failurealong the grain found in occasional logs. Sometimes these breaksare invisible until the wood is manufactured into the finishedarticle. The occurrence of this defect is mostly limited to thedense hardwoods, such as hickory and to heavy tropical species.It is the source of considerable loss in the fancy veneerindustry, as the veneer from valuable logs so affected drops topieces.

The cause of heart break is not positively known. It is highlyprobable, however, that when the tree is felled the trunkstrikes across a rock or another log, and the impact causesactual failure in the log as in a beam.

_Resin_ or _pitch pockets_ are of common occurrence in the woodof larch, spruce, fir, and especially of longleaf and other hardpines. They are due to accumulations of resin in openingsbetween adjacent layers of growth. They are more frequent intrees growing alone than in those of dense stands. The pocketsare usually a few inches in greatest dimension and affect onlyone or two growth layers. They are hidden until exposed by thesaw, rendering it impossible to cut lumber with reference totheir position. Often several boards are damaged by a singlepocket. In grading lumber, pitch pockets are classified assmall, standard, and large, depending upon their width andlength.

INSECT INJURIES[37]

[Footnote 37: For detailed information regarding insectinjuries, the reader is referred to the various publications ofthe U.S. Bureau of Entomology, Washington, D.C.]

The larvae of many insects are destructive to wood. Some attackthe wood of living trees, others only that of felled orconverted material. Every hole breaks the continuity of thefibres and impairs the strength, and if there are very many ofthem the material may be ruined for all purposes where strengthis required.

Some of the most common insects attacking the wood of livingtrees are the oak timber worm, the chestnut timber worm,carpenter worms, ambrosia beetles, the locust borer, turpentinebeetles and turpentine borers, and the white pine weevil.

The insect injuries to forest products may be classed accordingto the stage of manufacture of the material. Thus round timberwith the bark on, such as poles, posts, mine props, and sawlogs,is subject to serious damage by the same class of insects asthose mentioned above, particularly by the round-headed borers,timber worms, and ambrosia beetles. Manufactured unseasonedproducts are subject to damage from ambrosia beetles and otherwood borers. Seasoned hardwood lumber of all kinds, roughhandles, wagon stock, etc., made partially or entirely ofsapwood, are often reduced in value from 10 to 90 per cent by aclass of insects known as powder-post beetles. Finished hardwoodproducts such as handles, wagon, carriage and machinery stock,especially if ash or hickory, are often destroyed by thepowder-post beetles. Construction timbers in buildings, bridgesand trestles, cross-ties, poles, mine props, fence posts, etc.,are sometimes seriously injured by wood-boring larvae, termites,black ants, carpenter bees, and powder-post beetles, andsometimes reduced in value from 10 to 100 per cent. In tropicalcountries termites are a very serious pest in this respect.

MARINE WOOD-BORER INJURIES

Vast amounts of timber used for piles in wharves and othermarine structures are constantly being destroyed or seriouslyinjured by marine borers. Almost invariably they are confined tosalt water, and all the woods commonly used for piling aresubject to their attacks. There are two genera of mollusks,_Xylotrya_ and _Teredo_, and three of crustaceans, _Limnoria,Chelura_, and _Sphoeroma_, that do serious damage in many placesalong both the Atlantic and Pacific coasts.

These mollusks, which are popularly known as "shipworms," aremuch alike in structure and mode of life. They attack theexposed surface of the wood and immediately begin to bore. Thetunnels, often as large as a lead pencil, extend usually in alongitudinal direction and follow a very irregular, tangledcourse. Hard woods are apparently penetrated as readily as softwoods, though in the same timber the softer parts are preferred.The food consists of infusoria and is not obtained from the woodsubstance. The sole object of boring into the wood is to obtainshelter.

Although shipworms can live in cold water they thrive best andare most destructive in warm water. The length of time requiredto destroy an average barked, unprotected pine pile on theAtlantic coast south from Chesapeake Bay and along the entirePacific coast varies from but one to three years.

Of the crustacean borers, _Limnoria_, or the "wood louse," isthe only one of great importance, although _Sphoeroma_ isreported destructive in places. _Limnoria_ is about the size ofa grain of rice and tunnels into the wood for both food andshelter. The galleries extend inward radially, side by side, incountless numbers, to the depth of about one-half inch. The thinwood partitions remaining are destroyed by wave action, so thata fresh surface is exposed to attack. Both hard and soft woodsare damaged, but the rate is faster in the soft woods or softerportions of a wood.

Timbers seriously attacked by marine borers are badly weakenedor completely destroyed. If the original strength of thematerial is to be preserved it is necessary to protect the woodfrom the borers. This is sometimes accomplished by properinjection of creosote oil, and more or less successfully by theuse of various kinds of external coatings.[38] No treatment,however, has proved entirely satisfactory.

Fungi are responsible for almost all decay of wood. So far asknown, all decay is produced by living organisms, either fungior bacteria. Some species attack living trees, sometimes killingthem, or making them hollow, or in the case of pecky cypress andincense cedar filling the wood with galleries like those ofboring insects. A much larger variety work only in felled ordead wood, even after it is placed in buildings or manufacturedarticles. In any case the process of destruction is the same.The mycelial threads penetrate the walls of the cells in searchof food, which they find either in the cell contents (starches,sugars, etc.), or in the cell wall itself. The breaking down ofthe cell walls through the chemical action of so-called"enzymes" secreted by the fungi follows, and the eventualproduct is a rotten, moist substance crumbling readily under theslightest pressure. Some species remove the ligneous matter andleave almost pure cellulose, which is white, like cotton; othersdissolve the cellulose, leaving a brittle, dark brown mass ofligno-cellulose. Fungi (such as the bluing fungus) which merelystain wood usually do not affect its mechanical propertiesunless the attacks are excessive.

It is evident, then, that the action of rot-causing fungi is todecrease the strength of wood, rendering it unsound, brittle,and dangerous to use. The most dangerous kinds are the so-called"dry-rot" fungi which work in many kinds of lumber after it isplaced in the buildings. They are particularly to be dreadedbecause unseen, working as they do within the walls or inside ofcasings. Several serious wrecks of large buildings have beenattributed to this cause. It is stated[40] that in the threeyears (1911-1913) more than $100,000 was required to repairdamage due to dry rot.

Dry rot develops best at 75 deg.F. and is said to be killed by atemperature of 110 deg.F.[41] Fully 70 per cent humidity isnecessary in the air in which a timber is surrounded for thegrowth of this fungus, and probably the wood must be quite nearits fibre saturation condition. Nevertheless _Meruliuslacrymans_ (one of the most important species) has been found tolive four years and eight months in a dry condition.[42]Thorough kiln-drying will kill this fungus, but will not preventits redevelopment. Antiseptic treatment, such as creosoting, isthe best prevention.

All fungi require moisture and air[43] for their growth.Deprived of either of these the fungus dies or ceases todevelop. Just what degree of moisture in wood is necessary forthe "dry-rot" fungus has not been determined, but it isevidently considerably above that of thoroughly air-dry timber,probably more than 15 per cent moisture. Hence the importance offree circulation of air about all timbers in a building.

[Footnote 43: A culture of fungus placed in a glass jar and theair pumped out ceases to grow, but will start again as soon asoxygen is admitted.]

Warmth is also conducive to the growth of fungi, the mostfavorable temperature being about 90 deg.F. They cannot grow inextreme cold, although no degree of cold such as occursnaturally will kill them. On the other hand, high temperaturewill kill them, but the spores may survive even the boilingtemperature. Mould fungus has been observed to develop rapidlyat 130 deg.F. in a dry kiln in moist air, a condition under which ananimal cannot live more than a few minutes. This fungus waskilled, however, at about 140 deg. or 145 deg.F.[44]

The fungus (_Endothia parasitica_ And.) which causes thechestnut blight kills the trees by girdling them and has nodirect effect upon the wood save possibly the four or fivegrowth rings of the sapwood.[45]

The most common of the higher parasitic plants damaging timbertrees are mistletoes. Many species of deciduous trees areattacked by the common mistletoe (_Phoradendron flavescens_). Itis very prevalent in the South and Southwest and when present insufficient quantity does considerable damage. There is also aconsiderable number of smaller mistletoes belonging to the genus_Razoumofskya (Arceuthobium)_ which are widely distributedthroughout the country, and several of them are common onconiferous trees in the Rocky Mountains and along the Pacificcoast.

One effect of the common mistletoe is the formation of largeswellings or tumors. Often the entire tree may become stunted ordistorted. The western mistletoe is most common on the branches,where it produces "witches' broom." It frequently attacks thetrunk as well, and boards cut from such trees are filled withlong, radial holes which seriously damage or destroy the valueof the timber affected.

LOCALITY OF GROWTH

The data available regarding the effect of the locality ofgrowth upon the properties of wood are not sufficient to warrantdefinite conclusions. The subject has, however, been kept inmind in many of the U.S. Forest Service timber tests and thefollowing quotations are assembled from various reports:

"In both the Cuban and longleaf pine the locality where grownappears to have but little influence on weight or strength, andthere is no reason to believe that the longleaf pine from oneState is better than that from any other, since such variationsas are claimed can be found on any 40-acre lot of timber in anyState. But with loblolly and still more with shortleaf thisseems not to be the case. Being widely distributed over manylocalities different in soil and climate, the growth of theshortleaf pine seems materially influenced by location. The woodfrom the southern coast and gulf region and even Arkansas isgenerally heavier than the wood from localities farther north.Very light and fine-grained wood is seldom met near the southernlimit of the range, while it is almost the rule in Missouri,where forms resembling the Norway pine are by no means rare. Theloblolly, occupying both wet and dry soils, varies accordingly."Cir. No. 12, p. 6.

" ... It is clear that as all localities have their heavy andtheir light timber, so they all share in strong and weak, hardand soft material, and the difference in quality of material isevidently far more a matter of individual variation than of soilor climate." _Ibid._, p.22

"A representative committee of the Carriage Builders'Association had publicly declared that this important industrycould not depend upon the supplies of southern timber, as theoak grown in the South lacked the necessary qualities demandedin carriage construction. Without experiment this statementcould be little better than a guess, and was doubly unwarranted,since it condemned an enormous amount of material, and oneproduced under a great variety of conditions and by at least adozen species of trees, involving, therefore, a complexity ofproblems difficult enough for the careful investigator, andentirely beyond the few unsystematic observations of the membersof a committee on a flying trip through one of the greatesttimber regions of the world.

"A number of samples were at once collected (part of themsupplied by the carriage builders' committee), and the fallacyof the broad statement mentioned was fully demonstrated by ashort series of tests and a more extensive study into structureand weight of these materials. From these tests it appears thatpieces of white oak from Arkansas excelled well-selected piecesfrom Connecticut, both in stiffness and endwise compression (thetwo most important forms of resistance)." Report upon theforestry investigations of the U.S.D.A. 1877-1898, p. 331. Seealso Rep. of Div. of For., 1890, p. 209.

"In some regions there are many small, stunted hickories, whichmost users will not touch. They have narrow sap, are likely tobe birdpecked, and show very slow growth. Yet five of thesetrees from a steep, dry south slope in West Virginia had anaverage strength fully equal to that of the pignut from thebetter situation, and were superior in toughness, the work tomaximum load being 36.8 as against 31.2 for pignut. The treeshad about twice as many rings per inch as others from bettersituations.

"This, however, is not very significant, as trees of the samespecies, age, and size, growing side by side under the sameconditions of soil and situation, show great variation in theirtechnical value. It is hard to account for this difference, butit seems that trees growing in wet or moist situations arerather inferior to those growing on fresher soil; also, it isclaimed by many hickory users that the wood from limestone soilsis superior to that from sandy soils.

"One of the moot questions among hickory men is the relativevalue of northern and southern hickory. The impression prevailsthat southern hickory is more porous and brash than hickory fromthe north. The tests ... indicate that southern hickory is astough and strong as northern hickory of the same age. But thesouthern hickories have a greater tendency to be shaky, and thisresults in much waste. In trees from southern river bottoms theloss through shakes and grub-holes in many cases amounts to asmuch as 50 per cent.

"It is clear, therefore, that the difference in northern andsouthern hickory is not due to geographic location, but ratherto the character of timber that is being cut. Nearly all of thatfrom southern river bottoms and from the Cumberland Mountains isfrom large, old-growth trees; that from the north is fromyounger trees which are grown under more favorable conditions,and it is due simply to the greater age of the southern treesthat hickory from that region is lighter and more brash thanthat from the north." Bul. 80, pp. 52-55.

SEASON OF CUTTING

It is generally believed that winter-felled timber has decidedadvantages over that cut at other seasons of the year, and tothat cause alone are frequently ascribed much greaterdurability, less liability to check and split, better color, andeven increased strength and toughness. The conclusion from thevarious experiments made on the subject is that while the timeof felling may, and often does, affect the properties of wood,such result is due to the weather conditions rather than to thecondition of the wood.

There are two phases of this question. One is concerned with thephysiological changes which might take place during the year inthe wood of a living tree. The other deals with the purelyphysical results due to the weather, as differences intemperature, humidity, moisture, and other features to bementioned later.

Those who adhere to the first view maintain that wood cut insummer is quite different in composition from that cut inwinter. One opinion is that in summer the "sap is up," while inwinter it is "down," consequently winter-felled timber is drier.A variation of this belief is that in summer the sap containscertain chemicals which affect the properties of wood and doesnot contain them in winter. Again it is sometimes asserted thatwood is actually denser in winter than in summer, as part of thewood substance is dissolved out in the spring and used for plantfood, being restored in the fall.

It is obvious that such views could apply only to sapwood, sinceit alone is in living condition at the time of cutting.Heartwood is dead wood and has almost no function in theexistence of the tree other than the purely mechanical one ofsupport. Heartwood does undergo changes, but they are gradualand almost entirely independent of the seasons.

Sapwood might reasonably be expected to respond to seasonalchanges, and to some extent it does. Just beneath the bark thereis a thin layer of cells which during the growing season havenot attained their greatest density. With the exception of thisone annual ring, or portion of one, the density of the woodsubstance of the sapwood is nearly the same the year round.Slight variations may occur due to impregnation with sugar andstarch in the winter and its dissolution in the growing season.The time of cutting can have no material effect on the inherentstrength and other mechanical properties of wood except in theoutermost annual ring of growth.

The popular belief that sap is up in the spring and summer andis down in the winter has not been substantiated by experiment.There are seasonal differences in the composition of sap, but sofar as the amount of sap in a tree is concerned there is fullyas much, if not more, during the winter than in summer.Winter-cut wood is not drier, to begin with, thansummer-felled--in reality, it is likely to be wetter.[47]

The important consideration in regard to this question is theseries of circumstances attending the handling of the timberafter it is felled. Wood dries more rapidly in summer than inwinter, not because there is less moisture at one time thananother, but because of the higher temperature in summer. Thisgreater heat is often accompanied by low humidity, andconditions are favorable for the rapid removal of moisture fromthe exposed portions of wood. Wood dries by evaporation, andother things being equal, this will proceed much faster in hotweather than in cold.

It is a matter of common observation that when wood dries itshrinks, and if shrinkage is not uniform in all directions thematerial pulls apart, causing season checks. (See Fig. 27.) Ifevaporation proceeds more rapidly on the outside than inside,the greater shrinkage of the outer portions is bound to resultin many checks, the number and size increasing with the degreeof inequality of drying.

In cold weather, drying proceeds slowly but uniformly, thusallowing the wood elements to adjust themselves with the leastamount of rupturing. In summer, drying proceeds rapidly andirregularly, so that material seasoned at that time is morelikely to split and check.

There is less danger of sap rot when trees are felled in winterbecause the fungus does not grow in the very cold weather, andthe lumber has a chance to season to below the danger pointbefore the fungus gets a chance to attack it. If the logs ineach case could be cut into lumber immediately after felling andgiven exactly the same treatment, for example, kiln-dried, nodifference due to the season of cutting would be noted.

Water occurs in living wood in three conditions, namely: (1) inthe cell walls, (2) in the protoplasmic contents of the cells,and (3) as free water in the cell cavities and spaces. Inheartwood it occurs only in the first and last forms. Wood thatis thoroughly air-dried retains from 8 to 16 per cent of waterin the cell walls, and none, or practically none, in the otherforms. Even oven-dried wood retains a small percentage ofmoisture, but for all except chemical purposes, may beconsidered absolutely dry.

The general effect of the water content upon the wood substanceis to render it softer and more pliable. A similar effect ofcommon observation is in the softening action of water onrawhide, paper, or cloth. Within certain limits the greater thewater content the greater its softening effect.

Drying produces a decided increase in the strength of wood,particularly in small specimens. An extreme example is the caseof a completely dry spruce block two inches in section, whichwill sustain a permanent load four times as great as that whicha green block of the same size will support.

The greatest increase due to drying is in the ultimate crushingstrength, and strength at elastic limit in endwise compression;these are followed by the modulus of rupture, and stress atelastic limit in cross-bending, while the modulus of elasticityis least affected. These ratios are shown in Table XV, but it isto be noted that they apply only to wood in a much driercondition than is used in practice. For air-dry wood the ratiosare considerably lower, particularly in the case of the ultimatestrength and the elastic limit. Stiffness (within the elasticlimit), while following a similar law, is less affected. In thecase of shear parallel to the grain, the general effect ofdrying is to increase the strength, but this is often offset bysmall splits and checks caused by shrinkage.

The moisture content has a decided bearing also upon the mannerin which wood fails. In compression tests on very dry specimensthe entire piece splits suddenly into pieces before any bucklingtakes place (see Fig. 9.), while with wet material the blockgives way gradually, due to the buckling or bending of the wallsof the fibres along one or more shearing planes. (See Fig. 14.)In bending tests on wet beams, first failure occurs bycompression on top of the beam, gradually extending downwardtoward the neutral axis. Finally the beam ruptures at thebottom. In the case of very dry beams the failure is usually bysplitting or tension on the under side (see Fig. 17.), withoutcompression on the upper, and is often sudden and withoutwarning, and even while the load is still increasing. The effectvaries somewhat with different species, chestnut, for example,becoming more brittle upon drying than do ash, hemlock, andlongleaf pine. The tensile strength of wood is least affected bydrying, as a rule.

In drying wood no increase in strength results until the freewater is evaporated and the cell walls begin to dry[49]. Thiscritical point has been called the _fibre-saturation point_.(See Fig. 24.) Conversely, after the cell walls are saturatedwith water, any increase in the amount of water absorbed merelyfills the cavities and intercellular spaces, and has no effecton the mechanical properties. Hence, soaking green wood does notlessen its strength unless the water is heated, whereupon adecided weakening results.

[Footnote 49: The wood of _Eucalyptus globulus_ (blue gum)appears to be an exception to this rule. Tiemann says: "The woodof blue gum begins to shrink immediately from the greencondition, even at 70 to 90 per cent moisture content, insteadof from 30 or 25 per cent as in other species of hardwoods."Proc. Soc. Am. For., Washington, Vol. VIII, No. 3, Oct., 1913,p. 313.]

The strengthening effects of drying, while very marked in thecase of small pieces, may be fully offset in structural timbersby inherent weakening effects due to the splitting apart of thewood elements as a result of irregular shrinkage, and in somecases also to the slitting of the cell walls (see Fig. 25).Consequently with large timbers in commercial use it is unsafeto count upon any greater strength, even after seasoning, thanthat of the green or fresh condition.

[Illustration: FIG. 25.--Cross section of the wood of westernlarch showing fissures in the thick-walled cells of the latewood. Highly magnified. _Photo by U. S. Forest Service._]

In green wood the cells are all intimately joined together andare at their natural or normal size when saturated with water.The cell walls may be considered as made up of little particleswith water between them. When wood is dried the films of waterbetween the particles become thinner and thinner until almostentirely gone. As a result the cell walls grow thinner with lossof moisture,--in other words, the cell shrinks.

It is at once evident that if drying does not take placeuniformly throughout an entire piece of timber, the shrinkage asa whole cannot be uniform. The process of drying is from theoutside inward, and if the loss of moisture at the surface ismet by a steady capillary current of water from the inside, theshrinkage, so far as the degree of moisture affected it, wouldbe uniform. In the best type of dry kilns this condition isapproximated by first heating the wood thoroughly in a moistatmosphere before allowing drying to begin.

In air-seasoning and in ordinary dry kilns this condition toooften is not attained, and the result is that a dry shell isformed which encloses a moist interior. (See Fig. 26.)Subsequent drying out of the inner portion is rendered moredifficult by this "case-hardened" condition. As the outer partdries it is prevented from shrinking by the wet interior, whichis still at its greatest volume. This outer portion must eithercheck open or the fibres become strained in tension. If thisouter shell dries while the fibres are thus strained they become"set" in this condition, and are no longer in tension. Laterwhen the inner part dries, it tends to shrink away from thehardened outer shell, so that the inner fibres are now strainedin tension and the outer fibres are in compression. If thestress exceeds the cohesion, numerous cracks open up, producinga "honey-combed" condition, or "hollow-horning," as it iscalled. If such a case-hardened stick of wood be resawed, thetwo halves will cup from the internal tension and externalcompression, with the concave surface inward.

[Illustration: FIG. 26.--Progress of drying throughout thelength of a chestnut beam, the black spots indicating thepresence of free water in the wood. The first section at theleft was cut one-fourth inch from the end, the next one-halfinch, the next one inch, and all the others one inch apart. Theillustration shows case-hardening very clearly. _Photo by U. S.Forest Service._]

For a given surface area the loss of water from wood is alwaysgreater from the ends than from the sides, due to the fact thatthe vessels and other water-carriers are cut across, allowingready entrance of drying air and outlet for the water vapor.Water does not flow out of boards and timbers of its own accord,but must be evaporated, though it may be forced out of verysappy specimens by heat. In drying a log or pole with the barkon, most of the water must be evaporated through the ends, butin the case of peeled timbers and sawn boards the loss isgreatest from the surface because the area exposed is so muchgreater.

The more rapid drying of the ends causes local shrinkage, andwere the material sufficiently plastic the ends would becomebluntly tapering. The rigidity of the wood substance preventsthis and the fibres are split apart. Later, as the remainder ofthe stick dries many of the checks will come together, thoughsome of the largest will remain and even increase in size as thedrying proceeds. (See Fig. 27.)

[Illustration: FIG. 27.--Excessive season checking. _Photo by U.S. Forest Service._]

A wood cell shrinks very little lengthwise. A dry wood cell is,therefore, practically of the same length as it was in a greenor saturated condition, but is smaller in cross section, hasthinner walls, and a larger cavity. It is at once evident thatthis fact makes shrinkage more irregular, for wherever cellscross each other at a decided angle they will tend to pull apartupon drying. This occurs wherever pith rays and wood fibresmeet. A considerable portion of every wood is made up of theserays, which for the most part have their cells lying in a radialdirection instead of longitudinally. (See Frontispiece.) Inpine, over 15,000 of these occur on a square inch of atangential section, and even in oak the very large rays whichare readily visible to the eye as flakes on quarter-sawedmaterial represent scarcely one per cent of the number which themicroscope reveals.

A pith ray shrinks in height and width, that is, vertically andtangentially as applied to the position in a standing tree, butvery little in length or radially. The other elements of thewood shrink radially and tangentially, but almost nonelengthwise or vertically as applied to the tree. Here, then, wefind the shrinkage of the rays tending to shorten a stick ofwood, while the other cells resist it, and the tendency of astick to get smaller in circumference is resisted by the endwisereaction or thrust of the rays. Only in a tangential direction,or around the stick in direction of the annual rings of growth,do the two forces coincide. Another factor to the same end isthat the denser bands of late wood are continuous in atangential direction, while radially they are separated byalternate zones of less dense early wood. Consequently theshrinkage along the rings (tangential) is fully twice as much astoward the centre (radial). (See Table XIV.) This explains whysome cracks open more and more as drying advances. (See Fig.27.)

Although actual shrinkage in length is small, nevertheless thetendency of the rays to shorten a stick produces strains whichare responsible for some of the splitting open of ties, posts,and sawed timbers with box heart. At the very centre of a treethe wood is light and weak, while farther out it becomes denserand stronger. Longitudinal shrinkage is accordingly least at thecentre and greater toward the outside, tending to becomegreatest in the sapwood. When a round or a box-heart timberdries fast it splits radially, and as drying continues the cleftwidens partly on account of the greater tangential shrinkage andalso because the greater contraction of the outer fibres warpsthe sections apart. If a small hardwood stem is split whilegreen for a short distance at the end and placed where it candry out rapidly, the sections will become bow-shaped with theconcave sides out. These various facts, taken together, explainwhy, for example, an oak tie, pole, or log may split open itsentire length if drying proceeds rapidly and far enough. Initialstresses in the living trees produce a similar effect when thelog is sawn into boards. This is especially so in _Eucalyptusglobulus_ and to a less extent with any rapidly grown wood.

The use of S-shaped thin steel clamps to prevent large checksand splits is now a common practice in this country withcrossties and poles as it has been for a long time in Europeancountries. These devices are driven into the butts of thetimbers so as to cross incipient checks and prevent theirwidening. In place of the regular S-hook another of crimped ironhas been devised. (See Fig. 28.) Thin straps of iron with onetapered edge are run between intermeshing cogs and crimped,after which they may be cut off any length desired. The time fordriving S-irons of either form is when the cracks first appear.

[Illustration: FIG. 28.--Control of season checking by the useof S-irons. _Photo by U. S. Forest Service._]

The tendency of logs to split emphasizes the importance ofconverting them into planks or timbers while in a greencondition. Otherwise the presence of large checks may rendermuch lumber worthless which might have been cut out in goodcondition. The loss would not be so great if logs were perfectlystraight-grained, but this is seldom the case, most treesgrowing more or less spirally or irregularly. Large pieces crackmore than smaller ones, quartered lumber less than that sawedthrough and through, thin pieces, especially veneers, less thanthicker boards.

In order to prevent cracks at the ends of boards, small strapsof wood may be nailed on them or they may be painted. Thismethod is usually considered too expensive, except in the caseof valuable material. Squares used for shuttles, furniture,gun-stocks, and tool handles should always be protected at theends. One of the best means is to dip them into meltedparaffine, which seals the ends and prevents loss of moisturethere. Another method is to glue paper on the ends. In somecases abroad paper is glued on to all the surfaces of valuableexotic balks. Other substances sometimes employed for thepurpose of sealing the wood are grease, carbolineum, wax, clay,petroleum, linseed oil, tar, and soluble glass. In place ofsolid beams, built-up material is often preferable, as thedisastrous results of season checks are thereby largely overcomeor minimized.

TEMPERATURE

The effect of temperature on wood depends very largely upon themoisture content of the wood and the surrounding medium. Ifabsolutely dry wood is heated in absolutely dry air the woodexpands. The extent of this expansion is denoted by acoefficient corresponding to the increase in length or otherdimensions for each degree rise in temperature divided by theoriginal length or other dimension of the specimen. Thecoefficient of linear expansion of oak has been found to be.00000492; radial expansion, .0000544, or about eleven times thelongitudinal. Spruce expands less than oak, the ratio of radialto longitudinal expansion being about six to one. Metals andglass expand equally in all directions, since they arehomogeneous substances, while wood is a complicated structure.The coefficient of expansion of iron is .0000285, or nearly sixtimes the coefficient of linear expansion of oak and seven timesthat of spruce[50].

Under ordinary conditions wood contains more or less moisture,so that the application of heat has a drying effect which isaccompanied by shrinkage. This shrinkage completely obscures theexpansion due to the heating.

Experiments made at the Yale Forest School revealed the effectof temperature on the crushing strength of wet wood. In the caseof wet chestnut wood the strength decreases 0.42 per cent foreach degree the water is heated above 60 deg. F.; in the case ofspruce the decrease is 0.32 per cent.

The effects of high temperature on wet wood are very marked.Boiling produces a condition of great pliability, especially inthe case of hardwoods. If wood in this condition is bent andallowed to dry, it rigidly retains the shape of the bend, thoughits strength may be somewhat reduced. Except in the case of verydry wood the effect of cold is to increase the strength andstiffness of wood. The freezing of any free water in the poresof the wood will augment these conditions.

The effect of steaming upon the strength of cross-ties wasinvestigated by the U.S. Forest Service in 1904. The conclusionswere summarized as follows:

"(1) The steam at pressure up to 40 pounds applied for 4 hours,or at a pressure of 20 pounds up to 20 hours, increases theweight of ties. At 40 pounds' pressure applied for 4 hours andat 20 pounds for 5 hours the wood began to be scorched.

"(2) The steamed and saturated wood, when tested immediatelyafter treatment, exhibited weaknesses in proportion to thepressure and duration of steaming. (See Table XVI.) If allowedto air-dry subsequently the specimens regained the greater partof their strength, provided the pressure and duration had notexceeded those cited under (1). Subsequent immersion in water ofthe steamed wood and dried specimens showed that they wereweaker than natural wood similarly dried and resoaked."[51]